The invention relates to a blood purification apparatus comprising an ultrafiltration device, in which the ultrafiltration rate is controlled.
During a blood purification treatment by means of haemodialysis, haemodiafiltration or haemofiltration the patient""s blood is guided in an extracorporeal circuit through an ultrafiltration device and then returned into the patient""s body. In the ultrafiltration device body fluid and contaminants are removed from the blood by means of convection and in the case of haemodialysis and haemodiafiltration additionally by means of diffusion in order to compensate for the renal insufficiency. In the following the processes of extracorporeal blood purification are referred to as xe2x80x9chaemodialysisxe2x80x9d in accordance with the medical usage. Due to partial or complete failure of the renal function the patient""s body contains an excess of body fluid and contaminants dissolved therein at the onset of the treatment, which are removed via the blood circulation during the haemodialysis treatment. During the several hours of treatment there is the danger of serious blood pressure decreases (symptomatic hypotensions) as a result of the intensive removal of fluid. Such complications require immediate therapeutical measures for stabilisation of the blood pressure. It is common practice to infuse into the blood circulation a physiological saline solution (0.9% sodium chloride solution) as a substitute. This increases the blood volume and thus the blood pressure but simultaneously counteracts the necessary removal of fluid. The latter can be prevented by infusing smaller quantities of a highly concentrated (e.g. 20%) sodium chloride solution instead of larger quantities (e.g. 200-500 ml) of physiological saline solution. The considerably higher concentration of sodium ions in the solution as compared to that of the body tissue has the effect that by osmosis additional body fluid flows from the body tissue into the blood vessels. Infusion of a highly concentrated sodium chloride solution into the body requires however a more complex blood purification apparatus since an infusion pump necessitating a sophisticated control system must be provided.
U.S. Pat. No. 5,503,624 describes an infusion system with a controlling device in which the controlling device processes indistinct knowledge by means of linguistic variables and methods of the fuzzy logic. Several influencing components of different qualities are evaluated as indistinct knowledge and integrated in the automatic control of the infusion device taking into consideration their semantic meaning. This ensures control of the infusion of an additional fluid into the blood system during a dialysis treatment in such a way that the qualitatively different influencing components and the changes of their complex joint effects occurring during the treatment are recognised in accordance with their semantic meaning and included in the automatic infusion control.
European Patent EP 0 652 780 B1 describes a device for the prevention of hypotension in dialysis patients by replacing the infused hypertonic sodium chloride solution by an increased sodium concentration in the dialysis fluid (dialysate). For this purpose sodium is administered to the patient both via the dialysate as soon as a corresponding signal is generated by the patient or the medical personnel. However this is possible only when a blood pressure decrease entails symptoms which become aware to the patient. Sodium is dosed in accordance with predetermined values by means of a simple control system. Automatic control of the sodium dosage in dependence on the current blood pressure is not possible.
In the extracorporeal blood purification it is further common practice to adapt the ultrafiltration rate by so-called ultrafiltration profiles to the patient""s condition. However it is only possible to adjust the respective value of the ultrafiltration rate for various phases of the blood purification treatment prior to the onset of the treatment. If necessary, the preselected ultrafiltration rates can be manually changed during the treatment by the medical personnel. The apparatuses used for this purpose are simple control devices with ultrafiltration rates arbitrarily set by the medical personnel. They do not possess any sensors for detecting changes in the patient""s condition during the treatment.
Furthermore, it has been proposed to use sensors measuring the decrease in blood volume or the relative blood volume or the volume share of blood plasma or the volume share of the cellular blood constituents (haematocrit) to prevent the blood volume from excessively decreasing (hypovolaemia) during the haemodialysis treatment. The ultrafiltration rate is changed in dependence on the current measured values in such a way that the preselected default or limit values of these variables are met. These concepts are not suitable for an effective automatic blood pressure stabilisation since they do not directly measure the blood pressure behaviour and the aforementioned volume parameters take into consideration only part of the multi factorial causes which form the basis of the complex mechanisms of haemodialysis-induced hypotension.
It is therefore the objective of the present invention to provide a blood purification apparatus which automatically stabilises the patient""s changing blood pressure by controlling the ultrafiltration rate.
According to the invention an ultrafiltration device controls the ultrafiltration rate in dependence on the blood pressure and at least one blood pressure trend value derived from the blood pressure by means of a fuzzy control. The ultrafiltration rate is the fluid volume removed from the patient""s body per time unit via the membrane system of the ultrafiltration device. The ultrafiltration rate is not only preset by the control device but also automatically changed in dependence on the patient-related values. This showed that the blood pressure behaviour is a very informative parameter. During a dialysis treatment a blood pressure decrease (hypotension) may occur as a result of the removal of fluid from the blood system. The control device according to the invention can counteract such blood pressure changes in a suitable way by reducing the ultrafiltration rate in the case of excessive blood pressure decrease. Although an average value of the ultrafiltration rate is generally pre-determined for the entire treatment, this value is changed by the blood pressure-dependent control when the blood pressure pattern indicates a situation requiring intervention. A special advantage offered by this device is that such an intervention is a change in the ultrafiltration rate so that an infusion of sodium chloride solution is not necessary. Thus the high costs of an infusion pump can be saved. However this does not preclude that in the case of an extreme course of the treatment drugs are for example manually injected. In certain cases it may be reasonable to include in the control not only the ultrafiltration rate but also the increase of the electrical conductivity of the dialysis fluid or infusion of a highly concentrated or isotonic infusion solution by means of a programmable infusion pump.
The concept of the invention allows for further physiological input variables to be used for fuzzy control, e.g. the change of the patient""s blood volume as from the onset of the treatment (relative blood volume), or the change of the blood plasma volume (relative blood plasma volume), or the volume of the cellular blood constituents (haematocrit), the change of the ion concentration or of the electrical conductivity of the blood, the temperature of the blood or of the dialysis fluid.
The normal ultrafiltration profile used when the input variables of the fuzzy control do not meet abnormal values varies in accordance with an advantageous further development of the invention in dependence on the ultrafiltrate volume. The ultrafiltrate volume is the integral of the previous ultrafiltration rate and indicates the total fluid volume which has been removed from the body so far. Alternatively the ultrafiltration profile may depend on the time or any other physiological input variable, e.g. change of the ion concentration or the electrical conductivity of the blood. The ultrafiltration profile is in no case a preset rigid profile but is influenced by the measured blood pressure value and further input variables.
In accordance with a preferred further development of the invention the fuzzy controller receives the blood pressure value as well as a short-term and a long-term blood pressure trend value as input variables. Both trend values are integrated in the semantic domain blood pressure and allow judgement of the current blood pressure value including foresighted conclusions.